On this table, If I want to change "DEFAULT 'N'" on fd1 column and want to set 'N' to fd1 column of all rows.
This is possible when I use ALTER TABLE command like this.
==> ALTER TABLE test DROP fd1, ADD fd1 CHAR(1) NOT NULL DEFAULT 'N';

But, with pt-online-schema-change, this is not possible.

So, pt-online-schema-change tool skip some columns when copy rows or create trigger, then this request could be accomplished,
If pt-online-schema-change tool support option lIke below example. ^^

shell> pt-online-schema-change --skip-copy-columns="fd1" ...

I believe you can easily implement it. And some people really want to this feature.
Thanks again.

Verified. If we run those statements with pt-osc, it will change the column default value but will not set new value. With ALTER TABLE, it will change the column default values and set the new default value to all records.